Autoimmune patients undergoing B cell depletion therapy (BCDT) with Rituximab can enter clinical remission for years, while others experience only transient relief (months). Although we do not understand why some patients respond well to BCDT and others do not, it is clear that B cells are pathogenic in a subset of patients. Clinical data show that BCDT can be efficacious without significantly reducing autoantibody titer, suggesting that B cells cause pathology via by another mechanism. Therefore, it is important to identify the Ab-independent functions of B cells and understand how B cells performing these functions contribute to pathology in autoimmune disease. However, these efforts have been hampered because we understand so little about the Ab-independent effector functions of B cells. Our data show that one potentially important effector function of B cells is to secrete cytokines. Indeed, we know that cytokines made specifically by B lineage cells regulate humoral and cellular immune responses in vivo. Based on these data, we hypothesize that cytokine-producing B cell effectors (Beff) likely contribute to immune responses to pathogens as well as autoantigens and that Beff cells can be either protective or damaging depending on the immune microenvironment and the cytokine secreted by the Beff cells. However, without until recently, it has been difficult to experimentally address this hypothesis, as we were not able to track cytokine-producing Beff cells in vivo. Importantly, we have now identified a novel subset of IFN?-producing Beff cells that develop in response to influenza infection and are inappropriately expanded in a mouse model of SLE (YaaFcyRllb-/- or Yaa.R2 mice). These data now provide us with the unique opportunity to address our original hypothesis. Therefore goals of this proposal are to (i) determine the factors that regulate the development of IFN? Beff cells in infectious and autoimmune settings and (ii) identify the functional role(s) of these cells in immune responses directed against pathogens and autoantigens.